Add CovenantSQL support as optional database

Hey guys:

ZeroNet is an **amazing** project, I noticed that it's very slow to sync a site with a big database.
After reading how zeronet database sync and merge works, I think there is some room to optimize it.
I'm the developer of [CovenantSQL](https://github.com/CovenantSQL/CovenantSQL) which is a decentralized SQL database.
So, I wonder is it welcome to add a PR for CovenantSQL support as optional database?

And, here is a stupid forum demo using CovenantSQL: https://github.com/CovenantSQL/CovenantForum
CovenantSQL is a quite young project, we accept feature request to make CovenantSQL adopt ZeroNet better. Also any questions is welcome :-)

Start by reviewing ZeroNet's database sync and merge behavior and the CovenantSQL and CovenantForum examples linked in the issue. Clarify the optional-database integration scope and completion criteria with maintainers before proposing implementation.

索引モデルが issue の本文から書いたものです。